Method and apparatus for aligning discontinuous fibers

ABSTRACT

A method and apparatus for aligning discontinuous fibers (F 101, 212) is described. A feeder apparatus (20, 120, 220, 220A) is used to align the fibers in a horizontal plane for feeding to the aligning apparatus (40, 140, 240, 240A) providing an electrical (E) field to orient the fibers in one reselected direction. A support or conveyor (70, 170, 70) receives the aligned fibers. The method and apparatus provides composite products having improved physical properties because of the alignment. The fibers can be of different lengths and a mixture of different types to make composites with controlled microstructure and properties. The composite materials can be in the form of non-woven, discontinuous fiber reinforced thermoplastic stampable sheets with controlled fiber orientation distribution. The composites are useful for a variety of goods.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to an improved method and apparatus foraligning discontinuous fibers using spaced apart plates having anelectrical field between them and a feeder apparatus for aligning thefibers for movement between the plates. The feeder apparatus ischaracterized by preferably having an elongated slot or slots whichalign the long axis of fibers in a horizontal plane for subsequentperpendicular alignment between the plates, thereby preventing thefibers from moving with the long axis in the vertical plane andessentially falling in an uncontrolled manner. Further, the inventionrelates to the manufacture of a non-woven, discontinuous fiberreinforced thermoplastic sheet with controlled fiber orientationdistribution.

(2) Description of Related Art

Micro-mechanics models for composite materials predict that indiscontinuous fiber composites, modulus and strength values approachthat of the unidirectional continuous fiber composites, when the lengthof the fibers far exceed the critical fiber length and when the fibersare aligned in the direction of the load (Agarwal, B. D., et al.,Analysis and Performance of Fiber Composites, John Wiley & Sons, 121-131(1990); and Piggott, M., "Load Bearing Fibre Composites", PermagonPress, 72-79 and 83-89 (1980)). Aligned discontinuous fiber polymercomposites have a clear advantage over other composite material systemswith respect to overall performance and processability and arepotentially well suited for lightweight structural applications. Severaltechniques using hydraulic, electrical, magnetic or pneumatic means havebeen tried in the past to orient fibers in a preferred direction whileprocessing discontinuous fiber composites. However, these methods ofmaking aligned discontinuous fiber composites have met with limitedsuccess because the methodologies that were developed did not bringabout any significant reductions in fabrication times or costs.

There is a need for an improved method that can manufacturediscontinuous fiber composites with the fibers preferentially aligned inone direction using electric fields. There is a need for a methodamenable to high degree of automation and high speeds of operation,thereby reducing the cycle time needed to fabricate an orienteddiscontinuous fiber composite sheet or part.

Controlling the orientation of short/discontinuous fibers has been achallenge in the processing of composite materials be it in glass fibersheet molding compound (SMC) processing, resin injection molding (RIM)preform manufacture or injection molding. With increased realization ofthe performance payoffs of aligned discontinuous composites, severalattempts have been made to control fiber orientation. A review ofliterature relating to fiber alignment techniques that can be used inthe fabrication of aligned discontinuous fiber composites is presentedwhere the techniques are broadly classified into two categories viz.wet/slurry methods and dry methods. This categorization lends itself tothe general conclusion that wet/slurry methods are typically slower andless flexible in controlling fiber orientation as compared to the drymethods.

In the wet methods (Kacir, L., et al., Polymer Engineering and Science,Vol. 15, p. 525, 532 (1975); Vol. 17, p. 234 (1977); Vol 18, p. 45(1978); and Soh, S. K., Proc. 10th. Annual ASM/ESD Advanced CompositesConference & Exposition, Dearborn (1994)), the fibers are usually in awell agitated liquid suspension and a fiber mat is created by eitherdraining the liquid or raising a filter bed through the suspension.Control of fiber orientation is limited, but can be achieved to somedegree by guiding vanes or other means like electric fields when theliquid is dielectric (Knoblach, G. M., U.S. Pat. No. 5,057,253). Thedrawbacks in these processes is the introduction of an additional stepof drying the wet mat which reduces the speed of manufacturingdrastically, and secondly the fact that fiber mat preform has to befurther processed by reaction injection molding or polymer sheetimpregnation to result in a composite part. The mechanical performanceof the final part may also be sometimes lowered due to the presence ofvoids entrapped during the drying of the wet fiber mat.

Dry methods ((Talbot, J. W., et al., U.S. Pat. Nos. 4,664,856 (1987);4,113,812 (1978); and Peters, T. E., et al., U.S. Pat. No. 5,017,312(1991)) usually rely on electric fields or pneumatic means ((Ericson, M.L., et al., Composites Science and Technology 49:121-130 (1993)) tocontrol fiber orientation and are generally faster than the wetprocesses. Peters et al (U.S. Pat. No. 5,017,312) had developed thetechnology to manufacture oriented chopped glass fiber mats where acomplicated array of electrodes are embedded at the bottom of the matthat is being formed and also above the mat to force orientation of thefibers as they descend. Lack of proper understanding of the fiberelectro-dynamics resulted in a complicated orientation technique.Besides, the end product is a fiber preform which needs additionalprocessing of liquid resin molding, before it can become a finalcomposite part. Other patents of interest are U.S. Pat. Nos. 2,686,141to Sawyer, 4,111,294 to Carpenter et al, 4,347,202 to Henckel et al,4,707,231 to Berger and 5,017,312 to Peters et al. DuPont (Chang, I. Y.,et al., J. Thermoplastic Composite Materials, Vol. 4, p 227-252 (1991))introduced a long discontinuous fiber (LDF) thermoplastic compositeprepreg for aerospace applications that has fibers several inches longwhich makes it less flexible in molding complex shapes. Moreover, thestarting material is a continuous fiber impregnated prepreg. Anotherapproach for making oriented preformed glass mat reinforcedthermoplastics is by using spray-up techniques (Jander, M., Proc. 7thAnnual ASM/ESD Advanced Composites Conference, Detroit (1991)). Althoughthese techniques are fast, only stiff and long fiber bundles (1"-2") canbe oriented by this technique which generally results in poor matriximpregnation and inflexible fiber preforms.

The problem in the prior art is that the fibers are randomly introducedbetween the E-field plates. In general, this tends to result in a largefraction of fibers which are not oriented by the electrical field,particularly when the fibers have a longitudinal long axis which isessentially vertical. There is also a problem with rebound of the fiberson the surface where they are to be deposited. The result is fiberswhich are not properly aligned, regardless of the method used toovercome distortions of the plate adjacent electrical field to the mat.

The widespread use of high performance continuous fiber composites islimited to a great extent due to expensive fabrication costs, whilediscontinuous or short fiber composites form a major share of the fiberreinforced composites for non-structural applications, due to ease inprocessability. To optimize the balance between performance andprocessability in polymer composite material systems, a novel high speedmethod was developed which produces aligned discontinuous fibercomposites (ADF) using electric fields.

Objects

It is therefore an object of the present invention to provide a methodand apparatus for reliably positioning discontinuous fibers when using Efield plates by controlling the fiber electrodynamics. Further, it is anobject of the present invention to provide a method which is relativelyeasy to perform and amenable to high degree of automation. Furtherstill, it is an object of the present invention to provide a relativelysimple feeder apparatus which pre-positions the discontinuous fibersbetween the E-field plates.

Further, it is the object of this invention to use fibers coated withpolymeric powder to (i) provide a high speed methodology formanufacturing aligned discontinuous fiber composites; and (ii) provide amethod for manufacturing thermoformable or compression moldable aligneddiscontinuous fiber composite sheet.

These and other objects will become increasingly apparent by referenceto the following description of the invention and the drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a front view of the apparatus 10 of the present inventionparticularly showing a feeder apparatus 20 for aligning a stream ofdiscontinuous fibers F between parallel plates 41 and 41A provided withan electrical field in an alignment apparatus 40. FIG. 1A is a plan viewshowing voltage source 43 for plates 41 and 41A.

FIG. 2 is a plan view of the apparatus 10 along line 2--2 of FIG. 1,particularly showing the aligning of the fibers F by the feederapparatus 20.

FIG. 3 is a cross-sectional view along line 3--3 of FIG. 1, particularlyshowing the spacing of parallel plates 41 and 41A.

FIG. 4 is a cross-sectional view along line 4--4 of FIG. 3 particularlyshowing the alignment of the fibers F from the feeder apparatus 20 bythe electrical field between the parallel plates 41 and 41A.

FIG. 5 is a front cross-sectional view along line 5--5 of FIG. 2 showingthe ridges 25 in tray 21 of the feeder apparatus 20 for aligning thefibers F.

FIG. 6 is a schematic cross-sectional view of a feeder apparatus 20 andalignment apparatus 40 showing the rotational movement of the fibers Fby the electrical field as they drop to the bottom of the alignmentapparatus;

FIG. 6A is a circuit diagram 80 for providing a voltage betweenelectrical plates 41 and 41A and neutralizing field between the bottomedge of 41 and 41A and the top edge of 42 and 42A.

FIG. 7 is a front schematic view of an apparatus 100 including feederapparatus 120, alignment apparatus 140 and conveyor apparatus 170 forcontinuously producing a composite product 101.

FIG. 8 is a cross-sectional view of the composite product 101 producedby the apparatus 100 of FIG. 7 wherein, the fibers 102 are aligned inthe direction of travel of the conveyor apparatus 170. FIG. 8A is a sideview of the composite product 101 and FIG. 8B is a plan view of thecomposite product.

FIG. 9 is a front schematic view of an apparatus 200 including feederapparatus 220, alignment apparatus 240 and 240A and conveyor apparatus270 for continuously producing a composite product 201.

FIG. 10 is a cross-sectional enlarged view of the composite product 201produced by the apparatus 100 of FIG. 9 wherein the fibers 202 and 203are aligned in the direction of movement of the conveyor apparatus 270.

Figures 11A and 11B show high speed videographs of 1/2 inch long choppedE glass fibers setting in the orientation chamber between plates 41 and41A of FIG. 1 as in Example 1.FIG. 11A shows an E field of 400 KV/m andFigure 11B is without the E Field.

FIGS. 12A and 12B are scanning electron micrographs of chopped E-glassfiber coated with 10 micron nylon-12 powder (FIG. 12A) and uncoated(FIG. 12B) as in Example 1.

FIGS. 13A to 13D are graphs showing fiber orientation distributions ofaligned (E-field 400 KV/meter (black lines) and without the E-field withrandomly oriented fibers (open lines) as in Example 3.

FIG. 14 is a graph showing the effect of fiber alignment on tensilestrength of glass fiber nylon-12 ADF composites (Volume fraction (Vf) offibers is about 40%).

FIG. 15 is a graph showing the effect of fiber alignment on modulus ofglass-fiber-nylon 12 ADF composites (VF of fibers is about 40%)

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to a method for producing a compositematerial of aligned discontinuous fibers having a relatively long axisand relatively small cross-section which comprises: feeding the fibersoriented in a horizontal plane along the long axis in a space having anelectric field between spaced apart vertically oriented electrode platesso as to effectively align the fibers substantially horizontally withthe long axis between the electrode plates; depositing the alignedfibers on a surface; and bonding the aligned fibers together, preferablywith a polymeric material.

The present invention also relates to a method for producing acontinuous sheet of a composite material of aligned discontinuous fibershaving a relatively long axis and a small cross-section which comprises:introducing a flow of the discontinuous fibers coated with a polymerpowder or any combination of polymer powders into a space in a chamberhaving an open end away from where the coated fibers are introducedhorizontally aligned along the long axis to produce a suspension of thecoated fibers in the space provided by a chamber; providing an electricfield between spaced apart vertically oriented electrode plates adjacentthe open end of the space and adjacent to the open end of the space ofthe chamber so as to align the polymer coated fibers horizontally in adirection with the long axis between the electrode plates in thechamber; depositing the coated fibers in the direction on a movingsupport from the open end of the chamber; and heating the fibers on themoving support outside of the chamber so as to melt the polymer powderand connect the fibers together, thereby forming the sheet of thecomposite material.

The present invention also relates to an apparatus for providing a sheetof aligned discontinuous fibers having a relatively long axis and asmall cross-section which comprises: a vertically oriented chamberdefining a space with an open end; feed means for introducing a flow ofthe discontinuous fibers oriented in a horizontal plane into the spacein the chamber; spaced apart electrode plates in the space in thechamber and adjacent the open end of the chamber for providing anelectrical field in the space in the chamber to align the fibers withthe long axis between the plates; support means below the opening fordepositing of the aligned fibers.

Finally the present invention relates to an apparatus for producing acontinuous sheet of a composite material of aligned discontinuous fibershaving which comprises: a vertically oriented chamber defining a spacewith an open end; feed means for introducing a flow of the discontinuousfibers oriented in a horizontal plane coated with a polymer powder intothe space in the chamber; spaced apart electrode plates in the space inthe chamber and adjacent the open end of the chamber for providing anelectrical field in the space in the chamber to align the fibers withthe long axis between the electrode plates; a support means which ismoved by a drive means past the open end of the chamber so as tocontinuously deposit the coated fibers on the support means; and heatermeans away from the chamber through which the support is moved so as tomelt the polymer powder and connect the fibers together to form thecomposite material.

In its general form, the method preferably consists of three unitoperations; dry powder impregnation of short fibers with the polymermatrix and mechanical orientation of the fibers; fiber orientationcontrol using electric fields; and compression molding of the flexiblealigned discontinuous fiber mat. The method provides a quick andeffective means for fiber orientation and is characterized by absence ofsolvents or liquids during processing and flexibility of the ADF matwhich can be rapidly molded into a complex part. In particular, the highspeed manufacturing methodology and the performance of glass fiber-nylon12 ADF composites for structural applications are shown in the Examples.

The method and apparatus of the present invention can be used withdiscontinuous fibers of any type including natural, carbon, glass orpolymer fibers. The fibers do not have to be conductive to be orientedby the electrical field. If necessary, a sizing of any suitablecomposition can be applied to the fibers before or after they are cutinto the discontinuous fibers.

The discontinuous fibers have a relatively long length along a long axisand a relatively small cross-section, which is usually round. Usuallythe fibers are the diameter between about 10-3 and 10 mm and a lengthbetween about 0.3 and 5 cm. Preferably the cross-section is uniformalong the length. Usually the fibers are in the form of bundles ofbetween 50 and 3000 fibers. In the composite product the fibers can havedifferent lengths and can have different compositions. The compositematerial sheets so formed, if thermoplastic, can be stampable withheating. For tensile modulus properties to approach those of continuousfiber composites, the length of the fibers have to be greater than thecritical fiber length (for details refer to Agarwal et al and Piggott etal (cited above). Ten times the critical fiber length will result inapproximately 95% of the modulus values of the unidirectional continuousfiber reinforced composite. In simple terms this critical length dependson the interfacial shear strength between the fiber and the matrix andthe fiber diameter. For chopped glass fiber-nylon matrix composites, thelength at which the properties approach those of equivalent continuousfiber composites is approximately one (1) inch (2.54 cm).

The mat of fibers can be formed and then impregnated with a polymer orthe like to form a composite product and/or can be laminated with apolymer. Alternatively, the fibers can be deposited with a powdered orliquid polymer in order to form the composite product. U.S. Pat. Nos.5,310,582 to Vyakarnam and Drzal and 5,102,690 to Iyer, Drzal andJavaraman describe a method and apparatus for aerosolizing a powderusing a vibrating means, such as an acoustic speaker, which can be usedin the present invention and which is incorporated herein by reference.

The feeder preferably moves the fiber across a planar surface to alignthem in one direction which is parallel to a slot leading between theplates provided with the electrical field. This is preferablyaccomplished with a vibrator which moves the surface which is inclinedtowards the slot in a horizontal direction and by providing aligningridges periodically along the surface and parallel to the slot to aid inaligning the fibers.

The plates providing the electrical field are preferably charged at 100to 600 kilovolts per meter of spacing between the plates. The voltagedepends upon the fibers used. Above about 600 kilovolts corona dischargeoccurs; below 100 kilovolts there is not sufficient E-field. Preferably,the electrical field is produced by an alternating current (AC) source.

The fibers are deposited on a surface preferably a conveyor. Theconveyor can move back and forth or it can move in one direction or acombination of these movements can be used. All of this is well known tothose skilled in the art.

FIGS. 1 to 6 and 6A show a preferred apparatus 10 of the presentinvention including a feeder apparatus 20 (FIG. 2), an alignmentapparatus 40 for orienting the fibers F, and a conveyor apparatus 70 forsupporting the deposited fibers F. FIG. 6 is a schematic view ofselected of the parts of the apparatus 10 as shown in FIG. 1,schematically showing the orientation of the fibers F by the alignmentapparatus 40 from the feeder apparatus 20. FIG. 6A is a circuit 80 forthe apparatus 10 of FIG. 1.

The feeder apparatus 20 includes a specially designed tray 21 with anoscillator 22 which is magnetically operated (FMC, Homer City, Pa.)which produces high frequency oscillations of the tray 21. The tray 21is supported on the oscillator 22 by legs 23 and 23A connected by arm 24to one of the legs 23A. The tray 21 is provided with a series ofparallel ridges 25 and is inclined towards a slot 26 which is parallelto the ridges 25. The entrance 26A to the slot 26 forms an elongatefunnel to direct the fibers F (FIGS. 2 and 5) towards and through theslot 26. The tray 21 includes spaced apart vertical side walls 27 onopposed sides of the tray 21, an end wall 28 and a bottom 29 with theridges 25. The tray 21 has an angle of incline down towards the slot 26of about 5 degrees. As can be seen, the fibers F have an orientationafter passing through the slot 26 of the longitudinal long axis which isapproximately horizontal and parallel to the plane of the plates 41 and41A as shown in FIG. 4.

The alignment apparatus 40 is particularly shown in FIGS. 1, 3, 4, 6 and6A. FIG. 6 shows spaced apart parallel aligning plates 41 and 41A whichare provided with an electrical field by circuit 80. Neutralizing fieldsare provided by neutralizing field plates 42 and 42A. The fields forplates 41 and 41A and 42 and 42A are supplied by a voltage source 43(FIG. 6A). As shown in FIG. 6A, the plates 41A and 42A are grounded G;however, the plates 41 and 42 could be grounded G and the plates 41A and42A could be charged. Wires 44, 45, 46, 47 are provided to plates 41,42, 41A and 42A to provide the electrical connections. As can be seenfrom FIG. 6, the fibers F are re-oriented in a horizontal plane parallelto the drawing and perpendicular to the plates 41 and 41A by theelectrical feed between plates 41 and 41A. In reality the fibers may betilted at an angle along the longitudinal axis and FIG. 4 is merelyschematic.

FIG. 6A shows more detail for the electrical circuit of FIG. 6. A source48 of alternating circuit is transformed by voltage source 43. Avariable resistance or voltage divider 49 is provided for reducing thevoltage to plates 42. In the preferred embodiment the AC source 48 is at120 volts and 60 Hz so that the AC voltage source provides 25KV at 15mAmps. The voltage divider is variable between 0 and 25 KV in providinga voltage between plates 42 and 42A. The E-field intensity between theelectrode plates is varied by controlling the AC voltage source 48. Astandard VARIAC is used to control the source 48.

FIGS. 1 to 4 show the alignment apparatus 40 in more detail. Transparentinsulating shields 50 and 50A and 51 and 51A made of plexiglass(acrylic) are provided to support the plates 41 and 41A and 42 and 42Awhich mount by means of pegs (not shown) in holes 50B. Angled members41D provide an entry between the plates 41 and 41A. Insulating shields41B and 42B (FIG. 4) cover the plates 41, 41A, 42 and 42A. The holes 50Ballow for adjustment of the horizontal spacing of the plates 41 and 41Aand 42 and 42A. The plates 41 and 41A are mounted on insulating supports52 and 52A by means of insulating posts 54. Insulators 53 and 53A mountthe posts 54. Cylindrical rods 41C and 42C are provided to help minimizethe field intensity at the ends of plates 41 and 42, and between plates41A and 42A. A voltage probe 55 is connected to a multimeter 57. Box 56is connected to motor 76 for the conveyor 71. Wires 58 and 59 areconnected to and from box 60 to the oscillator 22 and power supply. Avariable switch 60A from oscillator 22 is provided, as is an on-offtoggle switch 60B. VARIAC 61 provides variable AC voltage from powersource P to high voltage transformer 43.

Conveyor apparatus 70 is shown in detail in FIGS. 1, 2, 3 and 4. Astandard tape conveyor 71 moves back and forth in a horizontal pathwhich is about 9 inches (22.5 cm) wide so that the aligned fibers aredeposited on sheet 72 mounted on supports 73 and 74 and base 75. Motor76 is controlled by controller 56 and connected by wire 78.

FIG. 7 shows an apparatus 100 for forming a composite product 101 asshown in FIG. 8. The composite product includes fibers 102 in a resinmatrix 103 laminated between non-conductive release or non-release veilor polymer film sheets 104 and 105. The fibers 101 are aligned parallelto the conveyor apparatus 170 by feeder apparatus 120 and alignmentapparatus 140. The conveyor apparatus 170 moves the aligned fibers 102.A mixture of discontinuous fibers 101 are fed to a hopper 121 andthrough opening 122. The fibers 102 are mixed with a powdered resin 106which is aerolized by a vibrator 123 in chamber 124 as described in U.S.Pat. No. 5,310,582 to Vyakarnam and Drzal and fed into tray 125 by chute126. Excess resin powder, that is not adhering to the fibers isprevented from entering the alignment apparatus 140 by filtering theresin out in the initial portion of tray 126 through a perforated screen128 in the tray 125 into a dust collector unit 129. This filtered outresin is subsequently recycled back into the process stream. Tray 125 isprovided with multiple slots each leading between plates 141, 141A,141B, 141C, 141D and 141E of alignment apparatus 140, which are similarto plates 41 and 42. The number of multiple slots is chosen based on thethickness of the final composite sheet and the production ratesrequired.

The conveyor 170 includes a belt 171 on rollers 172 and 173 which movethe sheet 105 through the aligned apparatus to receive the particles 106coated on the fibers 102. The sheet 105 is fed from roll 107. The powderparticles 106 and fibers 102 are heated by heater 174 to sinter them. Asecond roll 108 provides the sheet 104 which is laminated with thefibers 102 and resin 103 to form a composite 101 by roller pairs 175 and176. The composite 101 can be further compressed with heat and pressurebetween belts 177 and 178 on drive rollers 179 and 180 to form a morecompressed composite 101A.

FIG. 9 shows an apparatus 200 which is used to form a composite product201 with fibers 202 and 203 and resin 204 and 205 forming separatelayers laminated between sheets 206, 207 and 208. Dual feeders 220 and220A feed the fibers 212 between plates 221A and 221B and plates 221Cand 221D onto the sheet 208 which is fed from roll 209 and the sheet 207which is fed from roll 210. The layer 204 is then covered by the sheet206 from roll 211. The laminate is fed to belts 271 and 272 on wheels273 and 274 to form the composite product 201. A conveyor belt 275 onrollers 276 and 277 moves the sheet 208.

Conductive as well as dielectric fibers can be aligned in an electricfield as long as there is polarization of the fiber, which can beachieved when there is a difference between the dielectric constants ofthe fiber and the medium surrounding it. Both direct current (DC) andalternating current (AC) fields were evaluated to orient fibers. In DCfields, it was found that glass fibers charged and migrated to oneelectrode under a strong electrophoretic force which disturbed anyalignment achieved. Based on this observation and the fact thatpolarization times for the dielectric fibers are of the order of 10-4seconds or less, it was concluded that an AC field is the better choicefor alignment of fibers. Since the polarization times are very low, onecan orient fibers at a frequency of 60 Hz.

The ADF process development required the control of fiber orientationand a determination of the various parameters that influence it. Anexperimental setup was built to study the orientation behavior of fibersin electric fields of the apparatus of FIG. 1. A variety of glass fibersof lengths ranging from 1/8 to 2" (0.3 to 5 cm) have been experimentedin this setup. In principle, the experimental station consists of aparallel plate electrode geometry with provision for recording andanalyzing the fiber motions. A high speed video camera and Kodak EktaProEM high speed motion analyzer is used to study the electro-dynamics offiber motions. The key parameters investigated are the fiber alignmenttimes, fiber settling behavior and fiber orientation distributions(FODs).

The analytical model derived by Demetriades (J. Chem. Phys. 29:154(1958); and Arp. P. A., et al., Advances in Colloid and InterfaceScience, Vol. 12:295356 (1980)) to determine the alignment time (t) foran elliptical body to orient from an initial angle θ_(i) to θ_(f) alongthe electric field lines of intensity E_(O) is given by the followingequation. ##EQU1## This relationship shows that the alignment time is avery strong function of E-field intensity and the polarization functionP. P is in turn related to the ratio of dielectric constants of thefiber and the medium and the aspect ratio. Better fiber alignments arepossible at higher E_(O). However, it is to be noted that this equationcan only serve as a first approximation to determine the alignment timesof the fibers that can be used in the ADF process. This is because theequation is valid for fiber motions in the viscous flow regime (implyingvery small fibers or a very viscous medium), while the glass fibers thatwere used were relatively large. Besides, the fluid medium being airresults in fiber motions that lie beyond the viscous flow regime. Thisled to the development of a working model to predict alignment times forthe fibers that can be used in the ADF process. Glass fibers of lengthsranging from 1/8 2" with and without polymer powder impregnation havebeen successfully aligned in the experimental setup, as they settled inthe orientation chamber. High speed videography (1000 frames/sec) and,FODs of fibers deposited on the deposition plate confirmed high degreeof fiber alignment in A.C. fields of intensities 300 KV/m and greater(Figures 11A and 11B).

Based on the experiments conducted on the experimental setup and thetheoretical analysis, a prototype process was designed and developedthat can manufacture aligned discontinuous fiber mats in both batch modeas well as continuous mode. The alignment times of fibers were estimatedto determine the dimensions of the orientation chamber.

The mode of impregnating polymer matrix on fiber was by bringing incontact an aerosol of fine tribo-charged polymer powder and fibers, toprovide a uniform coating of particles on the fiber. Acousticaerosolization is an effective means to generate an aerosol of finepolymer powders of sizes less than 30microns (Iyer, S. R., et al.,Powder Technology 57(2), p 127-133 (1989)). This aerosol is entrainedthrough a nozzle where the fibers are introduced. The amount of matrixor binder that was deposited on the fibers was controlled by controllingthe particle concentration flux in the aerosol, which is directlyrelated to the matrix volume fraction in the composite. It is importantto note the uniform distribution of polymer particles around the fibers(FIGS. 12A and 12B) required the polymer to flow only locally over smalldistances, thereby reducing the polymer melt flow times during thecompression molding step (Padaki, S., et al., "Development of a Processand Consolidation Model for Powder Prepreg Composites", Proc. 10th.Annual ASM/ESD Advanced Composites Conference & Exposition, Dearborn(1994)). This improves matrix impregnation, minimized void formationduring consolidation and improved the mechanical performance of thepart. Powder impregnation makes compression molding of the ADF mat intoa final composite part a rapid step.

Dry powder impregnated fibers of lengths greater than the critical fiberlength were generated and fed into the electric field orientationchamber in a controlled manner as in FIG. 1. There was a maximumconcentration of fibers that could enter the orientation chamber inorder for the E-field to effectively orient the fibers. The maximum rateof production using the ADF process is a function of the critical fiberconcentration (C_(f)) in the orientation chamber. This concentrationC_(f) is in turn related to the aspect ratio of the fiber and the fiberorientation state. The lowest C_(f) is when the fiber orientation stateis in a 3-dimensional random state. However, the fibers were fed in apredominantly planar fashion. This predominantly planar orientationstate of the fibers increased the C_(f) compared to the random state.The orientation chamber dimensions were designed such that thesuspension of impregnated short fibers settle under terminal conditionsin a predominantly planar horizontal orientation before coming under theinfluence of AC electric fields of frequency 60 Hz. A combination ofE-field intensity and the processing conditions unique to the process,resulted in the ability to control the fiber orientation distributionsof the ADF mat that was formed on the moving veil at the bottom of theorientation chamber. The ADF mat formed on the moving veil was subjectedto heat to retain its integrity. The ADF process can operate undercontinuous mode to make a uniformly thick ADF mat or in a batch mode tolay up the fibers in a 3 dimensional orientation sequence which can becompression molded to a composite part.

The ADF processing methodology offers a solution to the problem ofreducing the cycle time in the fabrication of an aligned discontinuousstructural composite and lends itself for a highly automated process forthe following three reasons:

(i) controlled powder impregnation of the matrix gives immenseflexibility in the fiber volume fractions that can be obtained andeliminate the additional step of resin transfer molding as the casewould have been if a preform is the final product of the process;

(ii) a simple and rapid orientation technique using electric fields; and

(iii) by incorporating a technique to increase the concentration offibers that can be processed in the orientation chamber, therebyincreasing the production rate.

The prototype in Example 1 has a rated capacity of manufacturing 2lbs/hr of ADF mat. Conventional scale up factors, i.e. increasing thewidth of the orientation chamber from the existing 8 inches (20.3 cm) tothe desired width and using multiple orientation chambers to build upADF mat rapidly result in direct increases in production rates. Thesescale-up factors can be directly applied to the process withoutaffecting the principle of operation in any way.

The following are Examples illustrating the method of the presentinvention.

EXAMPLE 1

Processing Aligned and Random Discontinuous Fiber Composites UsingPolymer Powder Coating and E-Fields

Materials

The fiber-matrix system chosen for verifying the properties ofcomposites produced using the ADF process was chopped E-glass fibers offour different lengths 1/8, 1/4, 1/2 and 1 inch supplied byOwens-Corning Fiberglas (Granville, Ohio); and nylon 12 (ORGASOL) powdermatrix with a mean particle size of 10 microns supplied by Atochem (ElfAtochem North America, Inc., Philadelphia, Pa.).

Processing Conditions - For each fiber (F) length, aligned discontinuousfiber (ADF) panels were fabricated under two modes: (i) randomorientation of fibers which provided the base-line case for comparison;and (ii) aligned fibers using E-field of intensity 400 KV/m (FIGS.13A-13D). In order to fabricate an aligned discontinuous fiber panel,measured quantities of fiber (34.5 g) and the matrix powder (20.7 g)were used to make a composite with a final fiber volume fraction of 40%.Each ply was fabricated by continuously feeding the fibers and lettingthem fall through the alignment apparatus of FIGS. 1 to 6A with thepreferred fiber orientation (also see FIGS. 11A and 11B) . The orientedfibers were deposited on a TEFLON release film placed on sheet 72 whichtraverses back and forth, until the desired thickness is achieved. Thismat of fibers was then uniformly sprayed with the nylon 12 powder andlater exposed to an infra-red strip heater (not shown) that sintered theparticles in place making the ADF composite more handleable. ADFcomposite plies were stacked up and processed using the consolidationcycle described below.

Composite Fabrication

ADF composites that were produced in the method were carefullytransferred to a compression molding caul plate, without disturbing thefiber orientation distribution. A known number of plies were stacked updepending upon the final thickness of the composite part desired. Vacuumassisted compression molding was performed in a instrumented CarverPress (Fred S. Carver, Inc., Menomonee Falls, Wis.) to make a part withminimum voids. Polymer matrix material was characterized usingdifferential scanning colorimetry (DSC) to determine thermal transitionsand rheometrics to determine the polymer melt shear viscosity, beforeformulating the consolidation cycle. For making composites with a 40volume fraction fibers, the consolidation cycle consisted of heating theplies to 200° C. with vacuum applied starting at 100° C. A pressure of100 psi was applied at the point when the temperature reached 100° C. inthe heating cycle until the end of the cycle. The part is held at 200°C. for 5 minutes to ensure complete consolidation and then rapidlycooled to room temperature to minimize crystallization. Thisconsolidation cycle gave consistently good quality composites withminimal resin bleeding. It was verified that the fiber orientationdistribution (FOD) of the ADF mat was not disturbed during theconsolidation process.

Mechanical Testing - The mechanical properties of ADF composites weredetermined by conducting tensile tests using ASTM D638. The compositepanels, prepared as described earlier, were cut into dog bone specimensusing a C0₂ laser source of 360 Watts operated at a cutting speed of30"/min. The specimens were then carefully cleaned from the edges toavoid any crack initiation during the test. When comparing theproperties it should always be noted that the compatibility of theproprietary polyester sizing on the glass fibers and the nylon 12 matrixwas unknown. It may be possible to obtain better properties if a sizingmore compatible with the matrix is used.

                  TABLE 1                                                         ______________________________________                                        Effect of Fiber Alignment on the Mechanical Properties                        of Discontinuous Fiber Composites                                                       Tensile Modulus    Tensile Strength                                 Fiber     (Msi)              (psi)                                            Length    Random  Aligned    Random                                                                              Aligned                                    ______________________________________                                        1/8"      0.93    1.58        6,330                                                                               9,990                                     1/4"      1.10    1.77        9,580                                                                              15,640                                     1/2"      1.16    2.08       14,290                                                                              23,870                                     1"        1.32    2.60       17,740                                                                              32,950                                     ______________________________________                                    

Effect of Fiber Alignment The effectiveness of fiber alignment is veryclearly reflected by the performance of ADF composites compared to therandom base line cases for all the fiber length cases (Table 1).Improvements due to fiber alignment range from 70% to 97% in thestiffness values and 58% to 86% in the strength values. The tensileproperties follow expected trends as a function of fiber length. Table 2summarizes the improvements obtained in the tensile properties of ADFcomposites, due to fiber alignment. Improvements in stiffness range from60.9% to 97%, while improvements in strengths range from 57.9% to 85.7%.It was concluded that if modulus is the primary selection criteria thenfiber orientation becomes very critical and the length of the fiberplays a secondary role. On the other hand when strength values need tobe high a combination of good fiber alignment, long fiber length alongwith the use of a properly compatible fiber sizing, yielded betterstrength properties in ADF composites.

Effect of Fiber length

Theoretical predictions using the simplest case of a plastic materialsurrounding a reinforcing fiber, estimate that when the fiber lengthincreases to about 10 times the critical fiber length in a perfectlyaligned discontinuous fiber composite, the modulus and strength valuesapproach that of unidirectional continuous fiber composite of the samefiber volume fraction (Agarwal, B. D., et al., "Analysis and Performanceof Fiber Composites", John Wiley & Sons 71-103 (1993)). observing themodulus vs fiber length data (FIG. 15) the conclusion was that themodulus values were approaching asymptotic values when the fiber lengthincreases from 1/8" to 1". The properties of random discontinuous fibercomposites tended to level off from a fiber length of 1/2 inch onwards,while aligned discontinuous fiber composites had an increasing trendeven at the fiber length of 1 inch. The effect of fiber length was verydramatic in the case of strength values with an increase of about 300%when the reinforcing length of the fibers increased from 1/8" to 1".This was because in the case of composites with smaller length fibers,there is a higher density of fiber ends or stress ends which results incomposite failure at low stresses.

                  TABLE 2                                                         ______________________________________                                        Performance Improvements Due to Fiber Alignment in ADF                        Composites (V.sub.f = 40% approx.)                                            Fiber Length                                                                              Strength    Stiffness                                             (inch)      Improvement (%)                                                                           Improvement (%)                                       ______________________________________                                        0.125       57.9        69.9                                                  0.25        63.3        60.9                                                  0.5         67.0        79.3                                                  1.0         85.7        97.0                                                  ______________________________________                                    

EXAMPLE 2 Processing Aligned and Random Discontinuous Fiber CompositesUsing Polymer Film Impregnation and E-Fields

A moving veil with a layer of the polymer matrix film can be passedthrough the alignment apparatus 40. The deposited fibers can then becovered with another layer of the polymer film and heat treated to forma handleable composite or laminate with the desired fiber orientation.The thickness of such a material can be built by a series of alignmentapparatus as shown in FIGS. 9 and 10 and later compression molded,typical of a sheet lamination process to form a composite sheet. Theconsolidation cycle needed to process these composites is the same asdescribed above for the same polymer matrix.

EXAMPLE 3 Processing of Microstructure Controlled Hybrid DiscontinuousFiber Composites

Fiber Orientation Distribution

One of the objectives of developing the ADF process was to demonstratethe capability of making ADF composites with controllable fiberorientation distributions (FODs). The method utilized in obtaining theFOD consists of running the ADF process under the same conditions as onewould to manufacture a composite but this time not with the intention ofmaking a composite but recording a series of digital images using aPanasonic CCTV. A number of images of the fibers that are deposited onthe TEFLON release film were taken to give a statistically significantFOD. Global Lab image analysis software (Data Translation, Inc.,Marlboro, Mass.) was used to identify fibers and determine theirorientations in each image. Using the above, method a series of FODswere obtained for each of the chopped glass fiber mats that wereproduced under the two conditions: randomly oriented and aligned in theE-fields. Control of fiber orientation is by a combination of factors:electric field intensity and the hydrodynamics of fiber motion. Fiberdielectric constant and the fiber geometric dimensions are the two mostimportant material properties that affect the degree of orientation andthe alignment time in air. It had been discovered that with a carefulcombination of the geometric and the conductivity parameters, one canachieve different degrees of fiber orientation when subjected to thesame E-field conditions. Fiber orientation distributions were obtainedfrom a number of digital images for 1/4" and 1" long fibers at differentE-Field conditions. These FODs were further statistically reduced to afiber orientation parameter fp. A fp value of 1.0 indicates perfectlyaligned and a fp value of 0.0 is perfectly random, while a fp of 0.5 maybe considered as moderately aligned. For example, at an E-fieldintensity of 300 KV/m, it has been found that 1" fibers can get highlyaligned but 1/4" fibers are not even moderately aligned (Table 3).Numerous other combinations can be designed depending on the propertiesof the fibers and the desired effect needed in the final composite.

                  TABLE 3                                                         ______________________________________                                        Effect of Fiber Length and E-Field Intensity on the                           Degree of Fiber Orientation Distribution                                      E Field Intensity,                                                                            fp for 1/4"                                                                             fp for 1" Glass                                     KV/m            Glass Fibers                                                                            Fibers                                              ______________________________________                                        0      (Random Case)                                                                              0.04      0.07                                            300                 0.39      0.69                                            400                 0.58      0.73                                            500                 0.65      0.76                                            ______________________________________                                    

Hybrid Composite Processinq

A mixture of 1/4" and 1" fibers, with the same electrical conductivityfall through the same orientation chamber but were aligned to differentdegrees of orientation as indicated in Table 3. These hybrid compositescan then be fabricated using the same consolidation techniques asdescribed above in Examples 1 and 2. This effect has numerous advantageswhen a material form has to be fabricated that needs physical propertieslike thermal, electrical and mechanical to be different in the threedifferent axes.

In Examples 1 to 3, a novel high speed and low cost process wasdeveloped to manufacture aligned discontinuous fiber composites usingelectric fields. It has been demonstrated that the orientation ofdielectric fibers like glass fiber bundles (with and without polymerpowder coating) can be effectively controlled using electric fields(FIGS. 12A and 12B). ADF composites made of glass fibers and nylon-12matrix provide significant improvements in stiffness and strengthproperties with fiber alignment. ADF composites made with engineeringthermoplastics offer the stiffness to weight ratios typical of highperformance continuous fiber composites, the versatility ofthermoplastics and the durability and formability of sheet metals rankthem higher than the non-recyclable thermoset based SMC and SRIMcomposites (Nichols, D., "Glass-mat Thermoplastics Form StructuralParts", Advanced Materials & Processes, 10, p. 29-32 (1994)). The ADFcomposites process addresses the need for a process that can manufacturelightweight, low cost structural materials for the automotive anddurable goods industry.

The method provides the following:

1. A simple yet effective design of the orientation chamber forcontrolling fiber orientation in air. Three unique techniques wereincorporated in the design of the orientation chamber to counter theedge effects of the electric fields when it comes in contact with thefiber mat that was being laid up during continuous processing of ADFcomposites.

(i) Created an equal and opposite field under the high voltage electrodeby placing another set of electrode or plates 42 and 42A under themoving slide/mat/veil. The plates 42 and 42A neutralize the edge effectsof the high voltage electrodes or plates 41 and 41A, which otherwisedisturb the fiber orientation of the fibers that are aligned by theE-Field and that are deposited on the mat.

(ii) The bottom electrodes 42 and 42A were offset with respect to thetop electrodes to create E-field lines at an angle which helps inlanding the fibers on the moving slide/mat/veil with minimum bouncing.

(iii) Designed the edges of the electrodes which are closer to themoving mat with a conductive cap or rod 41C which has a cylindricalcross-section with a relatively large radius of curvature. Thecylindrical edge reduced the effective E-Field intensity at the edge,thereby reducing the edge effects and minimizing E-Field breakdowns.

2. Control of fiber orientation was by a combination of factors:electric field intensity and the hydrodynamics of fiber motion. Fibergeometric dimensions and the fiber dielectric constant are the two mostimportant material properties that effect the degree of orientation andthe alignment time in air. Conductive (e.g. carbon) as well asnon-conductive fibers (e.g. glass, aramid and other polymeric fibers)can be aligned by the ADF process. Fiber with and without polymer powderimpregnation can also be aligned in the ADF process. In the ADF processno pre-treatment of fibers is necessary in aligning the fibers evennonconductive glass fibers.

3. Developed a unique vibratory fiber feeder apparatus 20. Fibers withor without powder impregnation can be fed into the orientation chamberwithout entanglement. Fibers falling from the feeder apparatus 20 tendto fall with a planar orientation instead of a three dimensional randomstate which makes them align faster in the direction of the electricfield between plates 41 and 41A. Thus the feeder apparatus 20 indirectlyimproves the effectiveness of orientation chamber.

4. Polymer powder coating of the fibers and thereby controlling thematrix volume fraction in the final composite is one of the biggestadvantages of the ADF process. The combination of powder coating withelectric field alignment makes the method a high speed-low cost processfor making aligned discontinuous fiber composites. This step eliminatesthe additional step of resin transfer. The technology of powder coatingis well established in the group and extensively patented (U.S. Pat.Nos. 5,123,373 to Iver, Drzal and Javaraman and 5,310,582 to Vyakarnamand Drzal).

5. Alternatively, a moving veil with a layer of the polymer matrix filmcan be passed through the orientation chamber. The deposited fibers canthen be laminated with another layer of the polymer film and heattreated to form a handleable mat or laminate with the desired fiberorientation. The thickness of such a material can be built by a seriesof orientation chambers and later compression molded to a composite partas in FIGS. 7 and 9. Matrix film impregnation and processing of ADFcomposites also offers a high speed processing option.

6. High speeds of fabrication are possible and amenable to CAD/CAMrobotic manufacturing techniques. This can especially prove to be veryeffective in the case of a complex lay-up sequence often needed in alarge part. Based on the Computer-Aided Design (CAD), one can programthe field directions in the orientation chamber, the fiber and matrixpowder feed rate to lay up powder impregnated fibers in the desiredlocation with the desired fiber orientation with the orientation chamberbeing moved by a robotic arm.

7. Discontinuous fiber thermoplastic composites with controlled fiberorientation are a unique material form. A material form consisting of apolymeric matrix and inclusions such as fibers or elongated fillers(with aspect ratio greater than 1.0) where the orientation distributionof the inclusions can be controlled in a preferred fashion, therebycontrolling the composite property. Anisotropy in the property can becontrolled, unlike other processing methods like injection molding withfibers/fillers or compression molding of Sheet Molding Compounds (SMC)where the anisotropy in properties is manifested at undesired locationsas a result of the processing operation and not a feature that can becontrolled. Stiffness and strength of the ADF composites can becontrolled by changing the fiber orientation distribution in the processto address the need for structural composites.

Other properties that can be similarly controlled are the electricalconductivity and the thermal properties by a suitable selection of thefibers and the matrix. Material form with a controlled electricalconductivity can find applications in electromagnetic shielding.Material forms with a preferred fiber orientation may also become verycritical in bio-materials especially in the area of body implants. Thiscomposite also may have applications in the sports and recreationindustries where lightweight composite materials are needed.

8. Short fibers (as small as 1/8") can be aligned and incorporated inthe composite to give a material form that can be stamped into complexshapes. In the case of a material form with long fibers or continuousfiber composites it is not so easy to bend the fibers over small radiiof curvature. Moreover this material form is superior than wovencomposites in terms of better compressive strengths and inter-laminarshear strengths.

9. Composites which consist primarily of thermoplastic matrices whichcan be recycled can be produced. Thermosetting polymers can be processedthrough this route if the polymer is available in the powder form and inan uncured state.

It is intended that the foregoing description be only illustrative ofthe present invention and that the present invention be limited only bythe hereinafter appended claims.

We claim:
 1. A method for producing a composite material of aligneddiscontinuous fibers having a relatively long axis and relatively smallcross-section which comprises:(a) feeding the fibers horizontallyaligned along the long axis in a space having an electric field betweenspaced apart vertically oriented first electrode plates so as to alignthe fibers substantially horizontally along the long axis perpendicularto the first electrode plates; (b) depositing the aligned fibers on asurface; and (c) bonding the aligned fibers together.
 2. The method ofclaim 1 wherein in step (a) a feed plate is inclined downward towards ahorizontally oriented slot through which the fibers pass to align thefibers along the longitudinal axis and wherein a vibrating means for thefeed plate is used to move the fibers towards and through the slot. 3.The method of claim 1 wherein the AC electric field intensity is betweenabout 100 and 600 Kv/m.
 4. The method of claim 1 wherein a powderedpolymer is coated on the fibers in step (a).
 5. The method of claim 4wherein a feed plate is inclined downward towards a slot and whereinvibrating means for the feed plate is used to move the fibers towardsthe slot.
 6. The method of claim 1 wherein the slot is only slightlylarger than the diameter of the fibers.
 7. The method of claim 4 whereinthe fibers and the powdered polymer material are heated to bond thefibers together.
 8. The method of claim 6 wherein the surface is a firstsheet of a polymer on which the aligned fibers are deposited and whereina second sheet of the polymer is provided over the aligned fibers andfirst sheet and bonded to the first sheet with the fibers sandwichedbetween the first and second sheets.
 9. The method of claim 4 whereinthe fibers are aligned by being provided on a feed plate oriented in aninclined horizontal plane with a horizontally oriented slot throughwhich the fibers pass to horizontally align the fibers along the longaxis of the fibers.
 10. The method of claim 9 wherein a vibrating meansfor the feed plate is used to move the fibers towards and through theslot.
 11. A method for producing a composite material of aligneddiscontinuous fibers having a relatively long axis and a smallcross-section which comprises:(a) introducing in a space a flow ofdiscontinuous fibers coated with a polymer powder horizontally alignedalong the long axis to produce a suspension of the coated fibers in thespace provided by a chamber having an open end away from where thecoated fibers are introduced; (b) providing an electric field betweenspaced apart vertically oriented first electrode plates in the space andadjacent to the open end of the space of the chamber so as to align thepolymer coated fibers horizontally in a direction along the long axisperpendicular to the first electrode plates in the chamber; (c)depositing the aligned coated fibers in the direction on a surface fromthe open end of the chamber; and (d) heating the coated fibers on thesurface so as to melt the polymer powder and connect the fiberstogether, thereby forming the composite material.
 12. A method forproducing a continuous sheet of a composite material of aligneddiscontinuous fibers having a relatively long axis and a smallcross-section which comprises:(a) introducing a flow of thediscontinuous fibers coated with a polymer powder into a space in achamber having an open end away from where the coated fibers areintroduced horizontally aligned along the long axis to produce asuspension of the coated fibers in the space provided by a chamber; (b)providing an electric field between spaced apart vertically orientedelectrode first plates adjacent the open end of the space and adjacentto the open end of the space of the chamber so as to align the polymercoated fibers horizontally in a direction with the long axisPerpendicular to the first electrode plates in the chamber; (c)depositing the coated fibers in the direction on a moving support fromthe open end of the chamber; and (d) heating the fibers on the movingsupport outside of the chamber so as to melt the polymer powder andconnect the fibers together, thereby forming the sheet of the compositematerial.
 13. The method of claim 1 wherein vertically oriented secondelectrode plates are provided below the surface and adjacent the firstelectrode plates which second electrode plates act to neutralize thefield adjacent the surface.
 14. The method of claim 1 wherein verticallyoriented second electrode plates are provided below the surface andadjacent the first electrode plates which second electrode platesneutralize the field adjacent the surface and wherein the secondelectrode plates are offset vertically from the vertically firstoriented electrode plates.
 15. The method of claim 1 wherein verticallyoriented second electrode plates are provided below the surface andadjacent the first electrode plates which second electrode platesneutralize the field adjacent the surface, wherein the second electrodeplates are offset vertically from the vertically oriented firstelectrode plates, and wherein the plates have an end adjacent thesurface with a cylindrical cross-section.